The present invention relates to a method of extracting valuable minerals and elements from coal ash using both physical and chemical separation techniques. The method is particularly applicable for processing fly ash derived from entrained flow coal gasification and pulverized coal combustion operations.
For decades various researchers have investigated means for extraction of minerals and elements from fly ash. Some techniques have been promulgated to initiate new business opportunities while others are due solely to environmental concerns associated with landfill disposal of certain coal ashes.
Most of the state-of-the-art processes have concentrated on the recovery of one or perhaps two minerals or elements from fly ash either using physical separation techniques or chemical separation techniques, but rarely combining the two.
U.S. Pat. No. 4,319,988 (issued to Aldrich) describes a method for recovering magnetite (Fe.sub.3 O.sub.4) from coal ash using a combination of dry and wet magnetic separation. U.S. Pat. No. 4,121,945 (issued to Hurst et al) describes a physical recovery process using water to facilitate such separation. In the Hurst et al process, carbon, cenospheres, iron concentrate, enhanced pozzolans, and inert mineral fillers are produced. U.S. Pat. No. 4,242,313 (issued to Torma) describes a process for the recovery of alumina from fly ash via high temperature lime pretreatment to increase the depth of aluminum extraction, followed by treatment with sulfuric acid, solvent extraction of aluminum sulfate and subsequent calcination of the aluminum sulfate to yield alumina. U.S. Pat. Nos. 4,252,777 and 4,254,088 (issued to McDowell and Seeley) describe methods for improving the acid leachability of aluminum and other metals such as iron, titanium, uranium, and thorium from fly ash via high temperature alkali pretreatment of the ash. U.S. Pat. No. 4,243,640 (issued to Hill et al) describes a means of extracting aluminum from fly ash using a nitric acid leach process. U.S. Pat. No. 2,244,761 (issued to Hixson et al) describes a method for recovering aluminum via counter-current leaching with hydrochloric acid, followed by solvent extraction to remove fabric chloride from the acid leaching media. The leach solution containing aluminum chloride is then evaporated to form hydrated aluminum chloride crystals which are then thermally decomposed to form aluminum oxide.
Canon, Gilliam and Watson of Oak Ridge National Laboratories have done considerable work on the acid extraction of aluminum from fly ash using a variety of techniques such as direct acid leach and pressurized acid digestion, which process includes a hot sodium hydroxide pretreatment step to increase the acid leachability of aluminum from the fly ash. Burnett, Dunker and Murtha of Ames Laboratory have also done considerable work on various minerals recovery from fly ash techniques. Ames Laboratory developed the "HiChlor" Process, a chlorination process for the removal of metal chlorides from fly ash via high temperature reaction and vaporization.
The above processes, with the exception of the Aldrich patent, are not believed to have been commercialized. The economics of such processes must necessarily compete with other state-of-the-art methods that process other mineral feedstocks and which are already in commercial operation.
However, it is apparent that the earth's resources are not inexhaustible and that future generations must necessarily look toward unconventional (by today's standards) sources of raw materials to meet future needs. Millions of tons of coal ash are generated each year in all sections of the United States. Coal ash stands ready as an available resource for the production of many metals and compounds thereof and constitutes a preprocessed ore that is readily available.
It would thus be extremely advantageous to provide a method for the economic recovery of mineral values from coal ash. However, such a method must be economically attractive when compared to other state-of-the-art processes which are already in commercialization that use other types of feedstocks.
The present invention incorporates both physical and chemical separation techniques, maximizing saleable products and minimizing energy consumption and waste streams to yield an economically and environmentaly attractive minerals recovery process. It is designed in such a manner that process options exist for the production or deletion of certain products depending on coal ash quality, regional product markets and product selling prices. Whereas other processes have concentrated on physical processing or chemical processing to produce a few products from coal ash, no prior processes are believed to have maximized recovery of saleable products through the use of extensive physical separation techniques in conjunction with extensive chemical reaction and chemical separation techniques.